1. Field of the Invention
The present invention concerns a controller for a radio-frequency amplifier.
2. Description of the Prior Art
A radio-frequency amplifier (RF amplifier) serves to as identically as possible amplify a radio-frequency signal (RF signal) led to it in order to obtain a RF signal of greater power at its output. RF signals having pulse powers of 15 to 30 kW are necessary particularly for special medical examinations by means of magnetic resonance tomography (MRT). RF amplifiers are therefore employed in MRT systems for producing RF signals having power in this range. The RF signals are pulsed, meaning they require such power for a period ranging from a few μs to a few ms. Very precise pulsed power is necessary at the RF amplifier's output, especially in the case of functional MRI (magnetic resonance imaging), in order to produce high-quality medical images using the MRT apparatus. A pulse-repetition accuracy of the amplified RF signal in the order of approximately 1-4% can be achieved using a conventional transmission arrangement. The term “precise” in this context means that both the amplitude and phase of the RF signal have to meet exact specifications. To obtain accuracies of this order for the RF amplifier's RF output signal, the amplifier is oriented with regulating means.
A transmission arrangement for a magnetic resonance apparatus is known from DE 103 35 144 B3 that has controlling means for the amplitude and phase of the RF amplifier's RF output signal. The ratio between the RF amplifier's input and output power, namely the RF signal that is to be amplified to produce the amplified RF signal (called the actual amplification or actual gain), is determined by suitable detectors. The phase relationship between said two signals is also determined (called the actual phase difference). For example, an integrated gain and phase detector, for instance an AD8302 chip from the company Analog Devices, can be used for that purpose. A settable attenuator and settable phase control element are used in two separate control loops for keeping the RF output signal's output amplitude and output phase constant or at the desired ratio to the RF input signal, i.e., for setting a desired amplification (desired gain) or desired phase difference.
FIG. 4 shows an arrangement according to the prior art. A controller 202 is connected upstream of an amplifier 200, also called an RFPA (Radio Frequency Power Amplifier). A RF input signal 206 is fed into the arrangement at an input 204. This signal proceeds through the controller 202 and the amplifier 200 via the signal line 208, to exit the arrangement as an amplified RF output signal 212 at the output 210. The controller 202 has both a gain detector 214 and a phase detector 216.
Two measured variables, for both the RF input signal 206 and the RF output signal 212, are fed, via respective signal couplers 218 and 220 assigned to the input 204 and output 210 and via corresponding measuring leads 222 and 224, both to the gain detector 214 and to the phase detector 216. The gain detector 214 determines the actual amplitude amplification 226 (actual gain) and the phase detector 216 determines the actual phase difference 228 between the RF output signal 212 and RF input signal 206. High frequency energy (RF signals) in a range around 63 or 123 MHz, for example, is present on the measuring leads 222, 224.
The actual amplitude amplification 226 and actual phase difference 228, by contrast, as output signals of the gain detector 214 and phase detector 216, are low-frequency signals (LF signals). The actual amplitude amplification 222 and actual phase difference 228 are compared in comparators 230, 232 with a desired amplification 234 and desired phase difference 236 and appropriate correction signals are conveyed via control amplifiers 238, 240 to an attenuator 242 and a phase control element 244 in the signal line 208.
The signal paths all run separately from each other, so the attenuator 242 and the phase control element 244 consequently have mutually independent, separate control loops 246, 248. An integrated gain and phase detector, mentioned above, can alternatively combine the two discrete gain detector 214 and phase detector 216 components in the form of an IC 250, indicated in FIG. 4 by dashed outlining.
Especially in the case of MRT, for example, the input signals 206 and output signal 212 often have, in certain circumstances relatively short rise times, for example in the range of 10 □s. The attenuator 242 and phase control element 244 therefore must have rise times that are just as short, or preferably even shorter. Particularly with regard to the phase control element 244, this can cause problems in terms of implementation and costs.
Cascading of the attenuator 242 and phase control element 244 in the signal line 208 also causes a further problem because these are not ideal control elements. Thus, for example, the phase control element 244 can exhibit or cause amplitude errors and/or the amplitude control element, meaning the attenuator 242 can exhibit or cause phase errors. The two control loops 246, 248 as a result will no longer be mutually independent. In the worst case that can lead to undesired transient oscillations in the controller 202. Although it is conceivable to employ control elements of sufficiently high quality, or virtually ideal control elements that could reliably obviate the aforementioned problems, such elements are very demanding in terms of circuitry, thus adding to costs.
It is also known, instead of the cascaded amplitude control element 242 and phase control element 244, to provide an IQ control element (not shown) in the signal path 208 to the RF amplifier 200. The RF signal 206 requiring to be amplified is led to the IQ control element and split into two partial signals having a 90° phase offset. The partial signals then each traverse an I path and a Q path. The corresponding partial signal is weighted in the I path with an I factor and in the Q path with a Q factor. The partial signals are recombined in a summing unit and led to the RF amplifier 200. The IQ control element also influences the magnitude and phase of the RF signal 206 requiring to be led to the RF amplifier. However, multiplying the partial signals with the I and Q factor in each case influences only the amplitude of the partial signals and not their phase. Because the partial signals correspond, due to the 90° phase offset, to a real and imaginary component of a complex phasor (namely their sum), by the addition of the real and imaginary component, in the form of the IQ controller's output signal fed to the amplifier, the change in the respective amplitudes of the partial signals causes this total signal to be altered in amplitude or phase.
An IQ control element of this type embodied in analog form is readily able to provide the required rise times of far below 1 μs. For driving the IQ element it will, however, then be necessary to convert the intended phase and amplification changes, meaning the changes in amplitude in the RF signal (desired and actual values), into respective I and Q factors in keeping with the amplification factors for the real and imaginary component (partial signals in the partial paths). That could be done by A/D conversion and by means of digital calculations performed by a digital computer, but that would involve not inconsiderable computing time and effort. For that purpose, signals have to be converted from analog to digital, or vice versa, and corresponding computing operations performed. This is computing-intensive and expensive, especially in view of the timing constraints.